Tips for your approach, from the overall vision to the nuts-and-bolts of realising it.
In Depth Cine identifies their five steps for working on a documentary project (including managing your subjects and your own expectations):
- Identify the content
- Gear
- Visual Style
- Sound
- Coverage
Of particular note is this approach, as a single camera operator, to capturing both speaker and reaction (or shot / reverse-shot):

Further Viewing
Documentarian and cinematographer Mark Bone has an entire series on his approaches to and experience with documentary storytelling: